Black bars are black strips that show up either at the top and bottom or at left and right of a video. To understand the reason for presence of black bars we need to know about video aspect ratio. “Aspect Ratio” is ratio of width of the video to height of the video.
There are two common TV screen sizes:
                Conventional TV—aspect ratio 4:3        Widescreen TV—aspect ratio 16:9        
Video contents typically have either a 4:3 or 16:9 ratios. When the aspect ratios of the TV and video content are different, the results will be black bars. Depending on combination of above two ratios, there are two different types of black bars:                Horizontal black bars        Vertical black bars        
Viewing of a 16:9 video content on a 4:3 TV creates horizontal black bars. Video sequences of 16:9 resolutions are up scaled to 16:12 by inserting a black bar on the top and at the bottom of video frames. Then the up scaled 16:12 version is now suitable for viewing on 4:3 TV. This is shown in FIG. 3 where 301 is the main picture, and 302 is the black bar.
Viewing of a 4:3 video content on a 16:9 TV generates vertical black bars.
Video of 4:3 resolutions corresponds to 12:9. This 12:9 video is converted into 16:9 by inserting a black bar on the left and at the right side of video frames, thus enabling the video to be viewed on a 16:9 TV. This is shown in FIG. 4 where 402 is the main picture, and 401 is the black bar.
Note that whenever video content and TV aspect ratio are matched there will be no black bars.